This invention relates to a filter and in particular to a filter having adjustable poles wherein the poles are adjusted to suppress interference and improve signal-to-noise ratio.
It is known in the art to provide continuously adaptive filters for radio frequency (RF) applications. It is also known in the art to provide adaptive filters for telephony and image processing. For example, L type adaptive filters and Volterre adaptive filters are known. RF adaptive filters, at their simplest, permit selection of bandwidth by insertion or deletion of switchable capacitors, resulting in discrete choices of filter bandwidth. Non-discrete selection of bandwidth could be achieved using varactor diodes. However, filter selectivity for maximum rejection of co-channel interference is best achieved with a surface acoustic wave filter or with a crystal filter, both of which possess fixed bandwidths.
In the past, adaptive analog filters have been either parallel banks of bandpass filters with user-selectable finite filter bandwidth or electronically tuned filters with variable bandwidths controlled by an analog control voltage. One problem of the latter has been maintaining a steep filter skirt or high selectivity with changes in control voltage. Another problem has been compensating for receiver center frequency of the adaptive filter due to transmitter center frequency drift.
Adaptive filters have traditionally been used for digital applications at low frequencies. Digital sampling and channelization requirements have limited the use of adaptive filters in this context to audio and video applications. In addition, some digital applications require non-real time processing due to computational requirements of auto-correlation and cross-correlation matrices.
Therefore, an object of the invention is to meet the need to filter wideband jamming signals or offset narrowband co-channel commercial interference.
A further object of the invention is to use the presence of an audio pilot tone multiplexed within the baseband modulation spectrum as a discriminant which controls and determines the amount of bandwidth adaption.
Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein: